The present invention relates to camber measuring methods, and more particularly, to non-destructive methods of measuring camber in sheet metal.
Sheet metal normally is formed, transported, and sold in large coils. In order to fabricate goods from such sheet metal, it is first necessary to uncoil an appropriate length of sheet metal and cut a sheet to size. If the strip unrolled from the sheet metal coil were perfectly flat and straight, sheet metal sections in the shape of squares, rectangles or parallelograms could be formed simply by shearing the strip at an appropriate angle relative to the longitudinal dimension of the strip.
However, in addition to other anomalies such as nonhomogeneity and uneven thickness across its width and along its length, the strip of sheet metal in the coil contains "camber." Camber occurs in a sheet when one longitudinal side of the strip, or a portion of the strip, is longer than the opposing longitudinal side, which causes the strip to curve to the left or right relative to its central longitudinal axis. Although camber in a strip of sheet metal typically is very slight in comparison to the width or length of the strip, it is sufficient to cause problems when the strip is unrolled from the coil and sheared to form individual metal sheets. Many attempts have been made to eliminate camber from strips of sheet metal, but it almost inevitably appears in any given coil, and may cause the sheet to bend to the right or left in varying places along its length.
Since sheet metal having a minimal amount of camber is more expensive to produce than sheets having a greater amount of camber, it desirable for economic reasons to utilize a strip of sheet metal having as much camber as can be tolerated in the finished product. If the shearing blade, which cuts individual sheets from a strip, is not pivoted an appropriate amount to compensate for camber in a sheet being cut from an uncoiled strip, a "rectangle" of sheet metal results in which none of the corners are right angles, and the diagonals of the sheet are not equal in length. Such a sheet must be trimmed excessively to make it sufficiently "square" for use. This squaring generates an unacceptable amount of scrap.
However, if either the incoming sheet or the shear blade is pivoted so that the shear blade lies along a radius passing through the center of curvature of the cambered strip, a section of sheet metal can be cut from a strip which, although not perfectly square, is such that all the corners are right angles and the diagonals are equal in length. Provided the camber of the sheet section cut does not exceed reasonable limits, a sheet having this specific geometry is acceptable for most applications.
Unfortunately, since a single strip of coiled sheet metal may have camber in varying degrees to the left or right, if at all, it may necessary to reposition the shearing blade for each cut to form successive sheets of metal having the aforementioned geometry. Attempts have been made to determine the camber of a sheet on a continuous basis. For example, an apparatus having three photoelectric cells is used in which the light beams of the cells determine the physical edge of a strip of sheet metal, and from those three points an arc is determined. The shearing blade is positioned to lie along the radius of curvature of the arc to form the cut. Similarly, other devices incorporate three guide rolls which contact an edge of the sheet, and their positions are used to calculate an arc from which the radius of curvature line is derived.
A disadvantage with these types of devices is that the strip of sheet metal having camber usually is not formed such that the edges are in the form of perfect arcs, so that the derivation of the radius of curvature is an imperfect approximation. Furthermore, the guide rolls which guide the strip to the shear blade may deform the edges of the sheet and thereby throwoff the calculations made by these devices.
Accordingly, there is a need for an apparatus which is capable of determining the camber in strip sheet metal in the form of a radius of curvature of the strip on a continuous basis, so that camber in which the sheet veers to the right or left, if at all, can be detected. Furthermore, there is a need for a method of determining the camber of a strip of sheet metal without contacting the sides of the sheet which may be deformed by guide rolls and therefore present a false picture of the camber of the entire sheet.